My silent background push notifications were actually still working after I switched from certs to keys - the last time I know they worked was on 12/31/19.
I can send a background silent push notification to a particular FCM Token, instead of topics, through Firebase - that gets executed on the device. That means my APNS configuration in the app and on Apple's servers is a-ok, right?
I don't know how to figure out where this is failing, but I suspect it's something to do with Firebase Cloud Messaging - does that make sense, or am I missing something?
And is there any way I can debug this - any way I can see if the requests are getting to Apple from Firebase, and just not the devices?
It was a Firebase issue - Firebase support helped me figure it out.